1. Field of the Invention
The present invention relates to a charge pump circuit.
2. Description of the Related Art
In recent years, in electronics such as a mobile phone and a PDA (Personal Digital Assistants), are provided with devices requiring a drive voltage higher than a battery voltage, like an LED (Light Emitting Diode) used in a liquid crystal backlight. For example, in such a small information terminal, a lithium-ion battery is often used. An output voltage of the lithium-ion battery is approximately 3.5 V in usual condition and approximately 4.2 V in full charged condition, while an LED requires a higher voltage than a voltage of the battery. Where the higher voltage than the battery voltage is required, the battery voltage is stepped up by a charge pump circuit or a switching regulator, thus a voltage required for driving an LED is obtained.
The charge pump circuit generates an output voltage obtained by multiplying an input voltage by a given step-up rate. For example, if a battery voltage is 3 V and a step-up rate is two, an output voltage is fixed to 6 V. Accordingly, when a load circuit requires a drive voltage lower than 6 V, conventionally, a power transistor needs to be inserted into input side or output side of a charge pump circuit to adjust on-resistance and thereby adjust output voltage. For example, Japanese Patent Application (laid Open) No. 2000-262043 has disclosed a related technique.
Use of the technique disclosed in Japanese Patent Application (laid Open) No. 2000-262043, requiring a power transistor, increases the number of circuit components and a circuit area.